1. Field of the Invention
The present invention relates to a device for coupling two elements by means of screw engagement of male and female elements and to an antenna installation device which utilizes such a coupling device.
2. Prior Art
FIG. 18 is a cross sectional view of a conventional antenna installation device removably fastening the lower end of a rod-form automobile antenna to the upper end of an attachment base installed in a vehicle body.
A rod-form automobile antenna element 1 (whip antenna element) is provided with a joint 2 at the lower end. The joint 2 has a first screw element M provided integrally which has a male threaded part 3 formed on its outer surface by cutting or thread rolling. A beveled area 4 is formed on the external surface of the joint 2 so as to facilitate the screwing in or unscrewing of the antenna 1.
An attachment base 5 is made of an insulating material and is mounted on the vehicle body wall. A female screw element N, formed by cutting a female threaded part 6 (which can engage with the male threaded part 3) along the axis of a conductive part 7, is provided at the center of the attachment base 5. The end portion of a terminal rod 8 which is used for connecting an antenna feeder line is inserted into the lower end of the conductive part 7.
In this type of antenna installation device, the antenna element 1 is removably attached to the vehicle body wall by screwing the male screw element M of the joint 2 of the antenna element 1 into the female screw element N of the attachment base 5 in the direction indicated by the arrow in FIG. 18.
However, this type of antenna attachment device has some disadvantages. The male threaded part 3 of the male screw element M and the female threaded part 6 of the female screw element N are both formed by cutting or thread rolling using a tool such as a die or tap, etc. Thus, a considerable amount of work is required, and the cost becomes proportionally higher. Furthermore, unless finishing precision is very high, rattling, etc., tends to occur, and it becomes very difficult to maintain stable coupling. In addition, loosening tends to occur as a result of vibration, etc., during operation of the automobile. Accordingly, unless some device (e.g., a spring washer, etc.) is used to prevent such loosening, there is a danger that the coupled parts may separate and the antenna element 1 will fall out. Thus, the structure becomes more complex if a device for preventing loosening is added, resulting in that the installation is more complex.
Another type of antenna is also on the market, and this type of antenna provides a single-length whip antenna which is manufactured at low cost while providing excellent radio reception (which is a minimum requirement for such antennas). This antenna consists of a single conductive rod which is more or less matched to a quarter (1/4) wavelength of the FM wave band. When installed, the antenna is exposed outside of the vehicle. Thus, high tensile strength materials having a high recoil strength (e.g., high tensile strength stainless steel, etc.) are used so that the antennas can withstand loads which may be applied by obstructions while the vehicle is in motion and during washing.
When this type of whip antenna is mounted on a vehicle body in an assembly line, it occupies a large amount of dead space in the transport trucks used for transporting such vehicles. For this reason, the whip antenna is usually designed to be removable via screw coupling, etc., to an attachment base on the vehicle body.
This type of antenna, however, also has problems. Since it uses a material of high tensile strength, it has poor workability, and thread cutting is difficult. As a result, it is difficult to screw couple the antenna to the attachment base "as is". Accordingly, the antenna employs the structure as shown in FIG. 19. The joint 2 is fixed to the base end of the whip antenna element 1, and the male screw 3 attached to this joint 2 is screwed into the female screw 6 installed in the coupling part 5 of the attachment base 4. The joint 2 is made of a material which has a tensile strength lower than the whip antenna element 1 in order to insure good workability, and when this material is used it must be thoroughly tested in view of its strength. Accordingly, there is a limit in terms of manufacturing costs.
Another problem is that even if material of proven strength is used, screw coupling must be accomplished in a restricted space so that the dimensions of the male screw cannot be very large. As a result, if the load resulting from bending of the whip antenna element 1 in the direction indicated by the arrow in FIG. 19 is concentrated in the screw area, the root of the male screw 3 is likely to bend or break.